Hello everyone..I have a question, I want to make a chess robot with NXT Mindstorm for my undergraduate thesis. As we know that NXT brick only has 3 motors port. In this project, I need 4 motors port, because I'm still a student and doesn't have much money to buy multiplexer, how can I plug the other one motor to NXT brick (3 motors are in use) ? Is it able to use the sensorport (input) for motorport (output) ?Thanks for your attention.

You have a few options but none of them are free...1. Buy an EV3 this has four ports (but note that leJOS on the EV3 is stull under heavy development).2. Use more than one NXT and make them talk to each other via Bluetooth.3. Use some form of Motor Mux. that plugs into a sensor port.4. Create some other form of motor controller from scratch.

There is no magic solution that will let you plug a motor into a sensor port, sorry!

If you are looking for a good way to make a gearbox, I would recommend looking at the technic excavator (8043). It uses 4 motors, but has six functions. One motor is used to change the output, and the other 3 other motors now together have six functions. You could do the same with only 3 motors, one shifts and 2 become 4.